Isoxazoles reductive cleavage

Isoxazoles are susceptible to attack by nucleophiles, the reactions involving displacement of a substituent, addition to the ring, or proton abstraction with subsequent ring-opening. Isoxazolium salts are even more susceptible to attack by a variety of nucleophiles, providing useful applications of the isoxazole nucleus in organic synthesis. Especially useful is the reductive cleavage of isoxazoles, which may be considered as masked 1,3-dicarbonyl compounds or enaminoketones. 

C. Reductive Cleavage of Isoxazoles and of Their Hydrogenated Derivatives... 

Under the conditions used for the reductive cleavage of the O—N bond in isoxazoles, a similar reaction readily occurs with isoxa-zolines, e.g. on treatment with organomagnesium compounds and on catalytic hydrogenation.- Hydrogenolysis of the O—N bond (180 181 182 183) was used to elucidate the structure of isoxa-zolones from -ketoesters. -... 

Reductive cleavage of isoxazoles.2 Isoxazoles (1) are reductively cleaved to p-amino-a,p-unsaturated aldehydes or ketones when refluxed in moist acetonitrile containing Mo(CO)6 (0.2-1.0 equiv.). 

A common reaction intermediate (59) has been invoked79 to account for the isomerization of 5-alkoxyisoxazoles to azirine derivatives, and the reductive cleavage of 5-alkyl(aryl)isoxazoles to enamino ketones, in the presence of a catalytic amount of... 

Reductive cleavage of isoxazoles. In the presence of Mo(CO)6 and water (1 equiv.), isoxazoles are cleaved to / -amino-a,/ -enones in good yield.1... 

Reductive cleavage of isoxazoles.2 Sml2 (2 equiv.) in the presence of CH3OH as a proton source is an efficient reagent for reductive cleavage of the O—N bond in isoxazoles. 

Reductive cleavage of the isoxazole ring within 182 installed the requested carbonyl and hydroxymethyl functionalities giving densely... 

Substituted pyran-4-ones can be prepared from isoxazoles through reductive cleavage with Mo(CO)6 and acid-catalysed cyclisation of the generated enaminoketone <02TL3565>. 

When recent advances in the chemistry of isoxazoles were reviewed by Kochetkov and Sokolov1 in this Series in 1963 (and by Quilico2 in 1962), the main features of isoxazole chemistry had been established. Since then some important new discoveries have been made, but probably the most significant advances have concerned the exploitation of the known features of their chemistry in synthesis. Particularly noteworthy developments include the further application of the cycloaddition of nitrile oxides in the synthesis of isoxazoles (Section II,C and D), the use of isoxazolium salts in peptide synthesis (Section III,B,2), syntheses involving the products of reductive cleavage of isoxazoles as intermediates (Section III,D and E) and annelation reactions via deprotonation of alkylisoxazoles (Section 1II,E). 

Reductive cleavage with TMSCl/Nal of fused systems such as pyrano- and furo[3,4-cjisoxazole derivatives 18 gave predominantly polysubstituted isoxazoles 19, as key intermediates for further elaborations <03H1625>. Pyranoisoxazole derivatives 18a have been prepared by intramolecular 1,3-DC of nitrile oxides 21, obtained by treatment with n-... 

The reductive cleavage of the N—O bond of oximes is a reaction which has been widely used by synthetic chemists. When accompanied by reduction of the C=N bond the reaction leads to the formation of primary amines. Reactions of this type are described in Chapters 1.2-1.8, this volume. The reductive cleavage of cyclic oximes, particularly isoxazoles and 4,5-dihydroisoxazoles, has been used as a key step in several target syntheses of natural products. These cleavage reactions are covered in Chapter 3.8, this volume however, some of the methods which clearly bring about cleavage of the N—O bond before reduction of the C=N bond are also described in this section. The products of these reactions are A-unsub-stituted imines. Unless the imines are sterically protected or unless special precautions are employed in the work-up, the isolated products are usually carbonyl compounds formed by hydrolysis (Scheme 19). Reductive hydrolysis reactions of this type are also included here. 

---